Mr visualization of interventional devices

ABSTRACT

MR method for generating MR images of an object to be examined located inside an examination volume by means of an MR device and for simultaneously locating an interventional instrument inserted into the examination object, which instrument is equipped with a locating device. For the reconstruction of an MR image ( 20 ) MR signals together with device-delivered location signals from the examination volume are then recorded, after which the locating of the instrument takes place by the assessment of the image characteristics conditioned by the location signals. For making it possible to determine the position of the interventional instrument in an extremely rapid way, the invention proposes that the recording of the MR signals be effected by recording successively a plurality of MR partial data records ( 16, 17, 18 ) with respective incomplete and preferably non-cartesian sampling of the local frequency spectrum assigned to the examination volume. Two locating data records ( 21, 22 ) each are extracted from each of the MR partial data records ( 16, 17, 18 ). The actual locating of the instrument then takes place in that a difference image ( 33 ) is reconstructed from the two locating data records ( 21, 22 ).

The invention relates to an MR method for generating MR images of anobject to be examined located inside an examination volume by means ofan MR device and for simultaneously locating an instrument similarlylocated inside the examination volume, which instrument is equipped witha locating device, by means of which location signals can be generatedin the environment of the instrument, where MR signals together with thelocation signals from the examination volume are recorded for thereconstruction of an MR image, while the locating device issimultaneously switched to and fro between states of operation, andwhere the locating of the instrument takes place in that THE imagecharacteristics caused by the switching processes are assessed.

The invention also relates to an MR device for the execution of such amethod as well as a computer program for implementing such a method onan MR device.

Methods based on magnetic resonance (MR) have been gaining importanceincreasingly in recent times in the field of interventional radiology.The advantage that magnetic resonance has over the X-ray diagnosticsgenerally prevalent in this field is that neither the patient nor thedoctor carrying out the intervention is burdened with ionizingradiation. The magnetic resonance also has the advantage of a distinctlybetter soft portion contrast.

An important requirement for the efficient execution of interventionsmonitored by means of magnetic resonance is that MR images of theexamination volume of interest should be generated with a sufficientlyhigh image rate and that it should be possible to determine the positionof the instrument used for the intervention inside the examinationvolume sufficiently quickly and precisely.

An MR method of the type mentioned in the beginning is already knownfrom, for example, EP 0 930 509 A2. In the method already known,locating the interventional instrument takes place by means of alocating device positioned on the interventional instrument. Thislocating device has a coil arrangement and a modulation unit, while itis possible to modulate by means of a modulation unit a high-frequencysignal linked to the coil arrangement. While recording MR signals fromthe examination volume for the purpose of generating an MR image, thesignals contain as a location signal a modulated high-frequency signalof the coil arrangement caused by the locating device of theinterventional instrument, from which modulated high-frequency signalthe position of the instrument can be determined. According to themethod already known, sampling of the MR signals during image recordingis done with a sampling frequency double that of the frequency withwhich the signal radiated by the locating device is modulated. Due tothe Cartesian sampling of the local frequency spectrum at the time ofthe recording of the MR signals, this measure achieves the appearance ofthe signal generated in the locating device in an image area in thereconstructed MR image, in which area no image signals of the actualobject to be examined appear. It is thus possible to identify thelocating device and thereby to determine the position of theinterventional instrument.

The disadvantage of the method already known is that the locating of theinterventional instrument cannot take place until after a complete setof MR images has been recorded and after an MR image has beenconstructed from it. The frequency with which the locating takes placeaccording to the method already known thus exactly corresponds to theimage rate achieved. The time-dependent resolution that can be achievedin this manner in the locating and tracking of the interventionalinstrument is not sufficient in many applications. It is particularlydisadvantageous that the determination of the position of theinterventional instrument according to the method already known iscomparatively inaccurate, if the interventional instrument is movedduring the image recording.

For these reasons, it is an object of the present invention to providean MR imaging method which allows considerably accelerated locating ofan instrument present inside the examination volume. In so doing, therate, with which the position of the instrument is determined, should beindependent of the rate with which the actual MR images are generated.

Based on an MR method of the kind mentioned in the opening paragraph,this stated object is achieved in that the recording of the MR signalsand the location signals takes place by successively recording aplurality of MR partial data records, each with incomplete,non-cartesian sampling of the local frequency spectrum assigned to theexamination volume, where locating data records are extracted from eachof the MR partial data records, so that each of the locating datarecords is assigned to one of the various states of operation of thelocating device and where the locating of the instrument takes place byreconstructing a difference image from the locating data records.

According to the invention, considerable acceleration compared to theexisting state of the art technology is achieved in the locating of theinstrument, as the position of the instrument can be derived already byassessing only one MR partial data record. From the MR partial datarecord, which, according to the invention, comprises possibly only aminimum fraction of the image data required for the actual imagereconstruction, locating data records are first extracted for thispurpose, where each of the locating data records comprises exclusivelyMR signals and location signals, which are recorded in one of the statesof operation of the locating device. Then a respective image each isreconstructed from each of the locating data records, which hasunder-sampling artifacts due to the incomplete sampling of the localfrequency spectrum. The image signals originating from the object to beexamined, including the under-sampling artifacts, are almost completelycanceled in the difference image resulting from the subtraction of theseimages. In essence, difference signals remain, which are determined bythe switching sequences of the locating device. These difference signalsappear in image areas, which can be assigned to the position of theinstrument in a unique manner.

The instrument to be localized may be, as embodied in the invention, aninterventional instrument, such as an intravascular catheter or a biopsyneedle, or also markers made on the patient, which serve to determinethe position and orientation of the patient himself/herself within theexamination volume.

To be able to determine the position of the instrument within theexamination volume in a unique manner with the help of the differenceimage, according to the invention, the sampling of the local frequencyspectrum (also called k space) must be done effectively in anon-Cartesian manner. In case the sampling is done in a cartesianmanner, there are artifacts in the difference image due to thesub-sampling of the k-space, which are called ghosts, which distinctlyhamper a clear location. As a rule, additional information concerningthe position of the instrument would be necessary for Cartesian samplingto be able to make a clear assignment in the assessment of the imageinformation of the difference image for the purpose of locating of theinstrument. Such information can, however, be provided by using severalreceiving coils in parallel each with a different spatial sensitivityprofile for recording the MR signals, so that the position of theinstrument can be inferred with the help of the respective amplitudes ofthe location signals.

In the MR method according to the invention, the MR partial data recordscan be combined advantageously to a complete MR image data record, fromwhich the final MR image is then reconstructed. In this manner, the MRpartial data records recorded in a quick time sequence can be used forquickly and accurately locating the instrument, while then the recordedMR signals are at the same time used for reconstruction of the MR image.Thus the MR method as invented has no negative effect on the speed ofoverall image generation. According to the invention, only parts of thecomplete MR image data records are assessed for particularly fast andprecise locating.

Suitably, the switching frequency, at which the locating device isswitched to and fro between the different states of operation in the MRmethod according to the invention, is half the value of the samplingfrequency at which the MR signals are sampled during the recording ofthe MR partial data records. In this manner, the locating data recordscan be extracted from the MR partial data records, by recording datapoints successively in terms of time, and are assigned alternately toone of the two locating data records. This achieves at the same timethat the image signals of the images reconstructed from the locatingdata records originating from the object to be examined are more or lessexactly canceled in the difference image, so that the position of theinstrument can be readily determined. It is useful to have the samplingfrequency with which the MR signals are recorded match at least twicethe size of the examination volume, as the data points contained in theMR partial data records are split half each on the locating data recordsin the manner described before. The increase in the sampling frequencyeffects as much as possible a reduction of the sub-sampling artifactsinside the difference image, so that locating accuracy is not noticeablycompromised. Care must be taken to see that just an increase in thesampling frequency does not increase the measuring time.

It proves advantageous if the local frequency spectrum is sampledradially or spirally when the MR partial data records according to theinvention are recorded. The image artifacts remaining in the differenceimage due to the sub-sampling of the local frequency spectrum aredistributed over large image areas during radial or spiral sampling ofthe local frequency spectrum—unlike in cartesian sampling—so that no“ghosts” can arise and the position of the instrument can be determinedunambiguously without any problem with the help of the difference imagein spite of the sub-sampling. The locating of the instrument can bedone, for example, by finding pixels in the difference image, whosepixel brightness values exceed a threshold value that can bepredetermined. This threshold value should then be greater than theartifacts conditioned by the sub-sampling of the local frequencyspectrum in the difference image.

Suitable for executing the method according to the invention is an MRdevice with a primary field coil for generating a homogeneous staticmagnetic field in an examination volume, several gradient coils forgenerating magnetic field gradients in the examination volume, ahigh-frequency coil for generating high-frequency fields in theexamination volume and for recording MR signals from the examinationvolume, an instrument equipped with a locating device, by means of whichdevice location signals can be generated in the environment of theinstrument and with a central control unit for controlling the gradientcoils, the high-frequency coil and the locating device, as well as witha reconstruction and display unit for processing and displaying the MRsignals.

The method described above can be implemented on such an MR device bymeans of a suitable program control of the central control unit and/orthe reconstruction and display unit.

For example, a resonant circuit made from a micro coil and a capacitorand installed in the device may be considered a locating device, wherethe resonant circuit can be detuned by a control signal of the centralcontrol unit. The location signals of such a locating device arehigh-frequency signals, which are coupled into the resonant circuit andamplified by the resonant circuit. In a state of operation, the resonantcircuit is tuned to the resonance frequency of the MR device, so thatthe location signals can be recorded together with the MR signals andassessed for locating the instrument. In the other state of operation,the resonant circuit is tuned to a frequency deviating from theresonance frequency of the MR device, so that in this state ofoperation, no high-frequency location signal is generated by thelocating device. The detuning of the resonant circuit on switching overbetween the two states of operation can be done particularlyadvantageously by an optical method, as described for example in theabove-mentioned EP 0 930 509 A2.

Alternatively, the locating device may have an electrical conductorarrangement for generating an inhomogeneous magnetic field in theenvironment of the instrument. The inhomogeneous magnetic fieldspecifically destroys the coherence of the core magnetization in theenvironment of the instrument. As location signals are used in this casethe MR signals recorded from the environment of the instrument anddepending on the state of operation of the locating device.

The method according to the invention can be rendered available to theusers of the MR devices in the form of a suitable computer program. Thecomputer program can be stored on suitable data carriers, such as CD ROMor diskette, or can be downloaded from the Internet onto the controlunit of an MR device.

The invention will be described in more detail with reference to anexample of embodiment shown in the drawing, to which, however, theinvention is not restricted. In the drawing:

FIG. 1 shows an MR device according to the invention.

FIG. 2 shows a schematic representation of the MR method according tothe invention.

FIG. 1 shows an MR device as a block diagram. The device comprises aprimary field coil 1 for generating a homogeneous static magnetic fieldin an examination volume, in which a patient 2 is present. Aninterventional instrument 3 which may be an intravascular catheter, forexample, is inserted into the patient 2. A micro coil which is part of aresonant circuit is provided as locating device 4 in the interventionalinstrument 3. High-frequency location signals can be generated in theenvironment of the interventional instrument 3 by means of the microcoil in the manner described above. Furthermore, the MR device hasgradient coils 5, 6 and 7 for generation of magnetic field gradients indifferent spatial directions inside the examination volume. By means ofa central control unit 8, which is connected to the gradient coils 5, 6and 7 through a gradient amplifier 9, the time-dependent path of themagnetic field gradients inside the examination volume is controlled.Furthermore, the MR device shown here also comprises a high-frequencycoil 10 for generating high-frequency fields in the examination volumeand for recording MR signals from the examination volume. Thehigh-frequency coil 10 is connected with the central control unit 8 orwith a reconstruction and display unit 12 via a transceiver unit 11. TheMR signals processed by the reconstruction and display unit 12 can bedisplayed by means of a screen 13. The resonant circuit of the locatingdevice 4 in the MR device shown in the Figure can be detuned by means ofan optical signal. The locating device 4 is connected to a light source14 and a modulator 15 through an optical fiber running in theinterventional instrument 3, for modulation of the light from the lightsource 14. The light source 14 and the modulator 15 are controlled bythe central control unit 8, which controls the time-dependent path ofthe stored light signal and thereby the switching over of the locatingdevice 4 according to the method described above.

The method according to the invention is carried out on the device shownin FIG. 1 by means of a suitable programming of the control unit 8. Thereconstruction and display unit 12 is also involved in thereconstruction and assessment of the difference images for the purposeof locating the interventional instrument 3.

FIG. 2 depicts the processing of the MR signals by the method accordingto the invention. MR partial data records 16, 17 and 18 are recordedsuccessively in time, each with incomplete, non-Cartesian sampling, itis truss, of the local frequency spectrum assigned to the examinationvolume. The MR partial data records 16, 17 and 18 are then combined to acomplete MR image data record 19, from which an MR image 20 isreconstructed. The star-shaped continuous lines show that the recordingof the MR partial data records 16, 17 and 18 is carried out by means ofradial sampling of the local frequency spectrum in the depictedapplication. The sampling takes place each time in different radialdirections, such that all in all a complete sampling of local frequencyspectrum is available for the MR image data record 19. According to theinvention, two locating data records 21, 22 or, as the case may be, 23,24 or 25, 26 respectively are extracted from each of the MR partial datarecords 16, 17 and 18. This is done in such a manner that, for example,each of the two locating data records 21 and 22 is assigned to one ofthe two states of operation of the locating devices, i.e. for example,the MR partial data record 21 contains exclusively MR signals, which arerecorded while the resonant circuit of the locating device is tuned tothe resonance frequency of the MR device. The MR partial data record 22accordingly exclusively comprises MR signals, which are recorded withthe detuned resonant circuit. This may be achieved in that the switchingfrequency, with which the locating device is switched to and fro betweenthe two states of operation is half the sampling frequency with whichthe MR signals are sampled during the recording of the MR partial datarecords 16, 17 and 18. The locating data records 21 and 22 may then begenerated in that every second data point inside the MR partial datarecord 16 is included in either the locating data record 21 or in thelocating data record 22. At the same time, for minimizing thesub-sampling artifacts, the sampling frequency with which the MR signalsare recorded during the recording of the MR partial data record 16should be double the frequency actually required for sampling the localfrequency spectrum assigned to the examination volume. This ensures thatthe locating data records 21 and 22 contain complete data, at any rateas far as the radial sampling direction is concerned. In this manner,sub-sampling artifacts in images reconstructed from the locating datarecords can be minimized, which has a positive effect on the accuracyand clarity in the locating of the instrument. Individual intermediateimages 27, 28 or 29, 30 or 31, 32 are reconstructed from the locatingdata records 21, 22 and 23, 24 as well as 25, 26. For example, theintermediate images 27 and 28 differ from each other exclusively in thedifferent location signals generated by the locating device in the twodifferent states of operation. The radial direction of the sampling ofthe local frequency spectrum inside the two MR partial data records 21and 22 is in essence identical, as can be seen from the dotted linesarranged in a star shape in FIG. 2. This results in that the twointermediate images 27 and 28 have in a more or less identical mannercertain image artifacts caused by the incomplete radial sampling of thelocal frequency spectrum. The difference images 33 or 34 or 35 are thencalculated from the intermediate images 27, 28; 29, 30 and 31, 32. Imagesignals contained in the respective intermediate images are canceled inthe difference images 33, 34 and 35, where the image intensity isretained exclusively in the image areas 36, 37 and 38. This imageintensity can be attributed to different location signals in the twostates of operation of the locating device, such that the position ofthe interventional instrument can be determined from it.

It is clear from the FIG. 2 that the locating of the interventionalinstrument can be done several times—thrice, in the applicationshown—during the recording of a complete MR image 20. For this purpose,the MR partial data records 16, 17 and 18 are used, from which at thesame time the complete MR image data records 19 is composed for the MRimage 20.

1. An MR method for generating MR images of an object to be examinedlocated inside an examination volume by means of an MR device and forsimultaneously locating an instrument also located inside theexamination volume, wherein the instrument is equipped with a locatingdevice, by which location signals can be generated in the environment ofthe instrument, where MR signals together with the location signals fromthe examination volume are recorded for the reconstruction of an MRimages, while the locating device is simultaneously switched to and forbetween various states of operation, and wherein the locating of theinstrument takes place in that the image characteristics, caused by theswitching processes are analyzed, wherein the recording of the MRsignals takes place by successively recording a plurality of MR partialdata records with incomplete sampling of the local frequency spectrumassigned to the examination volume, where locating data records areextracted from each of the MR partial data records, so that each of thelocating data records is assigned to one of the states of operation ofthe locating device and where the locating of the instrument takes placeby reconstructing a difference image from the locating data records. 2.An MR method as claimed in claim 1, wherein the sampling of the localfrequency spectrum takes place during the recording of the MR signals ina non-cartesian way.
 3. An MR method as claimed in claim 1, wherein bycombining the MR partial data records, a complete MR image data recordis generated from which the MR image is reconstructed.
 4. An MR methodas claimed in claim 1 wherein the switching frequency, with which thelocating device is switched to and fro between the various states ofoperation, is half the sampling frequency with which the MR signals aresampled during the recording of the MR partial data records 16, 17 and18.
 5. An MR method as claimed in claim 2, wherein the local frequencyspectrum is sampled radially or spirally at the time of the recording ofthe MR partial data records.
 6. An MR method as claimed in claim 1,wherein locating the instrument is carried out by finding out pixels inthe difference image, the brilliance values of which pixels exceeding athreshold value that can be predefined.
 7. An MR device comprising aprimary field coil for generating a homogeneous, static magnetic fieldin an examination volume, a plurality of gradient coils for generatingmagnetic field gradients in the examination volume, a high-frequencycoil for generating high-frequency fields in the examination volume andfor recording MR signals from the examination volume, an interventionalinstrument which is equipped with a locating device, by which locationsignals can be generated in the environment of the interventionalinstrument to be recorded by means of the high-frequency coil, andcomprising a central control unit for driving the gradient coils thehigh-frequency coil and the locating device, and also comprising areconstruction and display unit for processing and displaying the MRsignals, wherein the control unit and/or the reconstruction and displayunit have a program control unit, by means of which an MR method isimplemented as claimed in claim
 1. 8. An MR device as claimed in claim7, wherein the locating device has a resonant circuit formed by a microcoil and a capacitor, which circuit can be detuned by means of a controlsignal from the central control unit.
 9. An MR device as claimed inclaim 7, wherein the locating device has an electrical conductorarrangement for generating an inhomogeneous magnetic field in theenvironment of the interventional instrument.
 10. A computer program foran MR device as claimed in claim 7, wherein a method as claimed in claim1 is implemented by the computer program on the control unit and/or onthe reconstruction and display unit of the MR device.
 11. A magneticresonance imaging device for imaging an object and an instrument locatedwithin the object, said device comprising: means for recording MRsignals as a plurality of MR partial data records; means for extractinglocating data records from each of the MR partial data records; meansfor assigning each of the locating data records a state of operation ofa locating device; and means for reconstructing a difference image fromthe locating data records in order to locate the instrument.
 12. Themagnetic resonance imaging device of claim 11 further comprising a meansfor combining the MR partial data records to form a complete MR datarecord, from which an MR image is reconstructed.
 13. The magneticresonance imaging device of claim 11, wherein the locating device has aswitching frequency that is approximately half of the sampling frequencyof the MR signals.
 14. A magnetic resonance imaging system for imagingan object and an instrument located within the object, said systemcomprising: an MR imaging device that produces MR signals based on asampling frequency; a locating device coupled to the instrument, whereinthe locating device has a switching frequency that is approximately halfof the sampling frequency of the MR imaging device; and a controllerthat creates locating data records from a portion of said MR signals,wherein said controller uses said locating data records to create adifference image in order to locate the position of the instrument.